1. Field of the Invention
This invention relates generally to capacitor discharge ignition systems for internal combustion engines.
2. Description of the Prior Art
The so-called Kettering ignition system has been commonly used for internal combustion engines for many years. The Kettering system is an inductive storage system basically consisting of the ignition coil, breaker contacts (points), the point capacitor, ballast resistor, and the DC power supply. The Kettering circuit is simple to understand, easy to service and generally dependable; however, it does have some objectionable characteristics. At low engine speeds, the point current may be too high resulting in short point life, the inductance of the primary winding of the ignition coil is typically too high for high speed operation, and the rise time of the secondary winding high voltage is typically too slow to insure adequate ignition voltage under adverse conditions. Furthermore, the Kettering circuit tends to produce a misfire condition when the ignition key or switch is turned-off at a time when the breaker points are closed.
Some improvements have been made in the Kettering system by replacing the breaker points with semi-conductors thereby eliminating the point wear and maintenance, and by using transistors to carry the heavy current of the primary winding of the ignition coil, using the breaker points to carry only the small control current for the transistors. While ignition coils having a lower primary winding inductance have been used for better high speed performance, the optimum value of the coil inductance is still a compromise when considering a range of engine speed and engine starting; improvement in one condition usually results in a sacrifice in other conditions, such as improving the high speed performance while lowering the low speed performance and making engine starting more difficult.
Numerous capacitor discharge ignition systems have been provided which have solved some of the problems inherent in the Kettering system but which have other problems of their own. One common form of capacitor discharge ignition system, as shown and described in U.S. Pat. Nos. 3,658,044, 3,704,699 and 3,714,507 employs a DC to AC inverter circuit which converts the battery voltage to high voltage alternating current, a rectifier, a capacitor connected in series with the primary winding of the ignition coil across the output of the rectifier, a silicon controlled rectifier also connected across the output of rectifier which, when turned-on, discharges the capacitor through the primary winding of the ignition coil, and a trigger circuit coupled to the gate element of the silicon controlled rectifier for providing a trigger signal to turn-on the silicon controlled rectifier in response to opening of the breaker points.
Some capacitor discharge ignition systems tend to misfire with varying battery voltage during cranking or when the ignition key or switch is turned-on or turned-off. Misfiring during cranking may damage the starter mechanism, and misfiring during key turn-on or turn-off will eventually cause conducting carbon paths to form between the terminals on the insulating surface inside of the distributor cap, such carbon paths also eventually causing misfiring. The most objectionable misfiring condition which occurs in prior capacitor discharge ignition systems is multiple firing, i.e., the condition when the next cylinder to be fired is fired prematurely along with the normally-fired cylinder. These two cylinders may be fired simultaneously or the misfiring may occur just after the spark break-off of the normal cylinder, or at a time at the end of the primary ringing with the system power capacitor. Simultaneous firing occurs with capacitor discharge ignition systems having excessive high voltage capacities at a time when the output voltage requirement for spark breakdown or corona ignition are at a minimum such as at low speed or idle with a hot, lean air-gas mixture. Such pre-ignition misfiring may occur after normal spark break-off since the secondary voltage of the ignition coil will rise to a very high level by reason of a very rapid rate of flux decrease or decay. If this high secondary voltage transient would cause a new spark to occur at the normal cylinder, there would be no harm done; however, the normal cylinder that has just fired may still be under sufficiently high compression to make spark break-down impossible even at that high voltage while, at the same time, the next cylinder in sequence is only under light compression and even with its large distributor cap air-gap, the high secondary voltage may be sufficient to cause pre-ignition by spark break-down or by corona ignition. Some capicator discharge ignition systems provide protection from multiple misfiring at cranking and low speeds by lengthening the spark duration or the duration of primary ringing; however, at high speeds, no protection against multiple misfiring is provided. Multiple misfiring may occasionally go unnoticed at moderate or high speeds however, in addition to lowering the efficiency, multiple misfiring may cause early engine failure such as blown piston heads, broken connecting rods or shortened distributor cap life. Furthermore, many prior capacitor discharging ignition systems tend to misfire during cranking due to loss of control of the triggering logic when the battery voltage drops to too low a level for properly engerizing.
Certain prior capacitor discharge ignition systems are not capable of high speed operation by reason of the inverter oscillation being stopped by the silicon controlled rectifier shorting the secondary winding of the inverter when the silicon controlled rectifier is turned-on, and also because the inverter is overloaded in the first portion of the capacitor charging cycle.
Other prior capacitor discharge ignition systems which use a large reservoir capacitor followed by a voltage doubling reactor which charges the power capacitor are not capable of high speed operation. In those systems, although the inverter oscillation is continuous, the reactor output falls off at high speeds because of slow inductance.
Despite the above-enumerated problems encountered in the use of prior capacitor discharge ignition systems, the capacitor discharge ignition system has numerous advantages as compared with the Kettering system such as lower point current with longer point life, both lower speed and higher speed capability, better cold weather starting, longer plug life by reason of less average plug current, better ignition even with fouled plugs and better fouled plug cleaning, and better ignition even with a defective ignition coil which has short-circuited coil sections. An outstanding feature of capacitor discharge ignition systems over the Kettering system is the rachet effect during the charging of the power capacitor; even though the battery voltage may dip or remain at a very low level just before the firing time, as may occur during cranking, the power capacitor will be charged at the highest battery voltage that occured from the time of the preceeding firing and thus will be able to produce a good spark. This is not possible with the Kettering or transistorized Kettering system since the maximum amphere-turns of the primary winding circuit of the ignition coil will decrease when the battery voltage is lower.